Last updated 10 August, 2001
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AEROSOLS What
are Aerosols? Atmospheric aerosols are very fine particles suspended in air. They are formed by the dispersal of material at the Earth's surface (primary aerosols), or by reaction of gases in the atmosphere (secondary aerosols). They include sulphates and nitrates from the oxidation respectively of sulphur dioxide and nitric oxide during the burning of fossil fuels, organic materials from the oxidation of volatile organic compounds (VOCs), soot from fires, and mineral dust from wind-blown processes. Natural aerosols, which also include sea salt and volcanic dust, are probably 4 to 5 times larger than man-made ones on a global scale, but regional variations in man-made pollution may change this ratio significantly in certain areas, particularly in the industrialised Northern Hemisphere. Although making up only 1 part in a billion of the mass of the atmosphere, they have the potential to significantly influence the amount of sunlight reaching the Earth’s surface, and therefore climate. Removal of most aerosols is mainly achieved by rainfall (wet deposition) and by direct uptake at the surface (dry deposition). Explosive volcanic eruptions however, can inject large quantities of dust and gaseous material, such as sulphur dioxide, high into the atmosphere (the stratosphere). Here, sulphur dioxide is rapidly converted into sulphuric acid aerosols. Whereas pollution of the lower atmosphere is removed within days by the effects of rainfall and gravity, stratospheric pollution may remain there for several years, gradually spreading to cover much of the globe. Like greenhouse gases, aerosols influence the climate. Atmospheric aerosols influence the transfer of energy in the atmosphere in two ways: directly through the scattering of sunlight; and indirectly through modifying the optical properties and lifetimes of clouds. The scattering of sunlight by aerosols is clearly demonstrated in the aftermath of a major volcanic eruption, when exceptionally colourful sunsets may be witnessed. The volcanic pollution results in a substantial reduction in the direct solar beam, largely through scattering by the highly reflective sulphuric acid aerosols. Overall, there is a net reduction of 5 to 10% in energy received at the Earth's surface. An individual eruption may cause a global cooling of up to 0.3oC, with the effects lasting 1 to 2 years. Estimation
of the impact aerosols have on longer-term global climate change
however, is more complex and hence more uncertain than that due to the
well-mixed greenhouse gases. This is largely because the
geographical distribution of aerosols is highly variable
and strongly related to their sources. The best estimates of
global cooling attributable
to man-made aerosols are based on computer models. These show that
the global cooling effect of man-made aerosols could offset the warming effect of
increased greenhouse gas concentrations by as much as 30%. The variable distribution of aerosols however, makes
calculation of a global average difficult. Nevertheless, it is
likely that aerosols may
slow the rate of projected global warming during the 21st century. More
about aerosols (NASA Website)
How do Aerosols Influence the Earth’s Climate? Tropospheric aerosols - including sulfates, soot, carbonaceous aerosols, biomass smoke, sea salt, and mineral dust - are recognized to affect global and regional climate. This phenomenon has been described in detail in recent publications from the Intergovernmental Panel on Climate Change, and the U.S. National Research Council “Aerosol Radiative Forcing and Climate Change” which highlight the following: (1) aerosols exert both a direct effect on climate - by reflecting and absorbing visible sunlight, and by absorbing and emitting infrared radiation - and an indirect effect on climate - by influencing the optical properties and persistence (i.e., lifetimes) of clouds; (2) on net, aerosols, including those emitted into the atmosphere as a result of human activities, reduce the amount of sunlight reaching the Earth’s surface, thereby exerting a cooling influence over large regions; (3) certain aerosols, most notably soot emitted from fossil fuel combustion, and wind-blown desert dust, absorb sunlight, resulting in a significant heating of the atmosphere in a manner somewhat analogous to the action of greenhouse gases; (4) because aerosols persist in the atmosphere only for short periods of time after they are emitted or formed (i.e., their lifetimes are short), and because aerosol sources vary greatly from location to location over the Earth, the impact of aerosols on the radiation impinging on the Earth’s surface (known as radiative forcing) is focused in particular regions and subcontinental areas, causing continental to hemispheric scale effects on climate patterns; and (5) there is substantial uncertainty in the magnitude and spatial/temporal distribution of the radiative forcing by aerosols. Indeed, the uncertainty in the radiative forcing induced by aerosols considerably exceeds that due to other known human influences on the climate system (for instance, emissions of the well-mixed greenhouse gases, such as carbon dioxide) presently included in prediction of climate change. This uncertainty limits our ability either to predict surface temperature changes associated with specified scenarios of greenhouse gas emissions, or even to unambiguously detect a greenhouse warming signal. Summary of the role of aerosols in climate
The
"Perfect Dust" Storm Jeff
Kiehl Science write-up Dr.
Joel Levy
“absorption” article on NOAA web page SeaWIFS
Aerosol Images from space Advanced
Info on Aerosols and Radiation Steve Schwartz 1994 Senate Testimony http://www.ecd.bnl.gov/steve/senate.html 3 VIS5D Captures Presented at ACE-Asia Briefing
by Dr. Gregory Carmichael, Dr. Itsushi Uno, Dr. David Streets,
Sarath Guttikunda, James Yienger, Narisara Thongboonshoo, James
Dorwart, Dr. Tang, Dr. Woo, and Phillip Keith
Images presented by Bill Collins of the National Center for Atmospheric Research and Phil Rasch, Climate Modeling Section, National Center for Atmospheric Research
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